North Korea has sentenced two South Koreans accused of spying to hard labor for life, South Korea's Yonhap news agency said on Tuesday, citing a North Korean state radio broadcast.

North Korea's highest court on Tuesday sentenced two South Koreans accused of spying to hard labor for life, its state media said, calling the punishment a lesson for those who conspire with Washington and Seoul.

The report of the sentencing came as the United Nations opened a field office in Seoul to investigate rights abuses in North Korea, a plan that has drawn anger from Pyongyang, which denies wrongdoing.

North Korea has accused the two men, Kim Kuk-gi and Choe Chun-gil, of working as spies for the South's National Intelligence Service (NIS) from the Chinese border city of Dandong. They were arrested in March.

The North's KCNA news agency said the two were convicted of conspiracy to overturn the state, espionage and illegal entry and of working under the control of the US and South Korean governments. The defense counsel requested leniency after the prosecution sought the death penalty, KCNA said.

"The crimes of the spies of the puppet intelligence agency prove that the US and the puppet South are the masterminds of political terror and kingpins of trickery and show what miserable plight awaits those who conspire with them," it said. The NIS has denied the accusations as "groundless." Kim and Choe said in interviews with CNN in May that they had spied for the South.

South Korea's unification ministry, which handles ties with the North, could not immediately confirm the North's broadcast.

North and South Korea are still technically at war after their 1950-53 conflict ended in a truce, not a treaty. The reclusive North, which regularly threatens to destroy the US in a sea of flames, has also been slapped with heavy Western sanctions over its nuclear and missile programs.

North Korea's state media has accused one of the two prisoners of running an "underground church" and spreading foreign information.

In addition to the two, Pyongyang is holding a South Korean man with a US green card who was a student at New York University, as well as a South Korean missionary who was sentenced to life with hard labor last year for espionage and setting up an underground church.

Last year, Pyongyang released three detained Americans including Kenneth Bae, a Korean-American missionary who had been held for two years.